In existing air-conditioning apparatuses such as multi-air-conditioning apparatuses used for buildings, for example, outdoor devices (outdoor units) that are heat source devices installed outside the buildings and indoor devices (indoor units) installed inside the buildings are connected by pipes to form refrigerant circuits through which refrigerants circulate. Air is heated or cooled by utilizing heat transfer or heat removal of the refrigerants to heat or cool the spaces to be air-conditioned.
In a case where a heating operation is performed with such a multi-air-conditioning apparatus used for a building as described above at an outside air temperature below approximately −10 degrees C., the low-temperature outside air and the refrigerant undergo heat exchange. Thus, the evaporating temperature of the refrigerant decreases, and the evaporating pressure decreases accordingly.
Consequently, the density of the refrigerant that is sucked into a compressor decreases and the refrigerant flow rate decreases, resulting in an insufficient heating capacity of the air-conditioning apparatus. In addition, as the density of the refrigerant that is sucked into the compressor decreases, the compression ratio increases, causing an excessive increase in the temperature of the discharge refrigerant of the compressor. Thus, problems such as deterioration of refrigerating machine oil and damage to the compressor occur.
In order to address the problems described above, an air-conditioning apparatus has been proposed (see, for example, Patent Literature 1) which is configured to inject a two-phase refrigerant into a region with intermediate pressure in the compression process of the compressor to improve the density of the refrigerant to be compressed to increase the refrigerant flow rate so that sufficient heating capacity can be achieved when the outside temperature is low to reduce the discharge temperature of the compressor.
The technology described in Patent Literature 1 utilizes the fact that when the saturation temperature of a high-pressure refrigerant supplied to a load side heat exchanger becomes higher than or equal to the temperature of the indoor air, heat is transferred from the high-pressure gas refrigerant to the indoor air so that the refrigerant is liquefied and becomes a two-phase refrigerant, and injects the two-phase refrigerant into a region with intermediate pressure in the compression process of the compressor to reduce the discharge refrigerant temperature of the compressor.
Patent Literature
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2008-138921 (FIG. 1, FIG. 2, etc.)
When the outside air temperature is below approximately −10 degrees C. the temperature of the space to be air-conditioned where an indoor unit is installed also decreases correspondingly. That is, for a period of approximately 5 to 15 minutes immediately after the start of the air-conditioning apparatus, the saturation temperature of a high-pressure refrigerant supplied to a load side heat exchanger provided in the indoor unit is lower than the indoor air temperature. Thus, in the heating operation, even if a high-pressure refrigerant is supplied to the load side heat exchanger, the high-temperature, high-pressure gas refrigerant will not be liquefied in the load side heat exchanger.
In the technology described in Patent Literature 1, therefore, when the air-conditioning apparatus operates under a low outside air temperature condition, the gas refrigerant is injected into the compressor, resulting in a reduced effect of suppressing the increase in the temperature of the refrigerant discharged from the compressor. In addition, as the outside air temperature decreases (for example, −30 degrees C. or less), the density of the refrigerant to be sucked into the compressor decreases, resulting in an increase in the increase range of the discharge refrigerant temperature of the compressor.
Specifically, in the technology described in Patent Literature 1, the discharge refrigerant temperature of the compressor temporarily excessively increases to approximately 120 degrees C. or higher before the high-pressure refrigerant becomes higher than or equal to the indoor air temperature, causing problems of “deterioration of refrigerating machine oil” and “damage to the compressor due to wear of a slider in the compressor caused by the deterioration of the refrigerating machine oil”.
In the technology described in Patent Literature 1, furthermore, the adoption of a method in which the speed of the compressor is reduced to reduce the rotation speed to suppress an increase in the discharge refrigerant temperature of the compressor may hinder a smooth increase in the speed of the compressor, causing a problem of increasing the time taken to achieve sufficient heating capacity and reducing user comfort.